This invention generally relates to an electrophotographic printing machine, and more particularly concerns an apparatus for housing an original document therein.
Generally, electrophotographic printing machines are provided with a transparent platen upon which the original document is supported. An optical image of the original document is created and recorded as an electrostatic latent image upon a photoconductive surface. The printing machine is usually provided with a cover to prevent extraneous light from entering the optical system during the imaging process and to reflect light rays through the machine optical system.
Frequently, it is desired to mask selected portions of the original document so that the copies do not contain this information. Hereinbefore this has been accomplished by securing opaque sheets to the platen so as to prevent the illumination of selected portions of the original document. Similarly, it may be desirable to electrically create an original document. In the past, this has been accomplished through the utilization of a cathode ray tube which creates letters or other indicia and directly projects the foregoing onto a charged photoconductive surface recording an electrostatic latent image thereon.
Various types of platens have been employed in the art, most of these being transparent. With the advent of liquid crystalline technology, it has become feasible to employ a liquid crystalline platen. Liquid crystals are fluids that are partially ordered so that they have some of the optical properties of crystal. Although they have been known for nearly a century, their recent application to display systems dates back to the discovery in 1968 that they have readily usable electro-optical properties. This is disclosed in greater detail in an article entitled "Liquid-Crystal Display Devices" by G. H. Heilmeier; Scientific American, April, 1970. At present, there are two types of crystals used in displays; dynamic-scattering liquid crystals and field-effect liquid crystals. Only dynamic-scattering liquid crystals will be discussed in detail as these are more readily adaptable for employment as a platen. Dynamic-scattering liquid crystals are clear in the absence of an electric field. When an electric field is applied thereto, they turn cloudy and scatter light. This effect is like frosting a piece of glass. Devices can be made transmissive for rear of front lighting applications.
Liquid crystals have been employed as color filters for modulating light rays passing therethrough. This is described in greater detail in U.S. Pat. No. 3,569,614 issued to Hanlon in 1971. Generally, a liquid crystal cell includes a strip of microglass paper impregnated with liquid crystal material sandwiched between two substrates. Each of the substrates has a conductive material placed thereon. This is described in greater detail in U.S. Pat. No. 3,746,426, issued to Masi, in 1973. It is evident that the employment of a transparent platen for supporting an original document which may be made selectively opaque would be highly desirable in an electrophotographic printing machine. The foregoing may be used to mask selected portions of the original document or to create an original document thereon.
Accordingly, it is the primary object of the present invention to improve platens supporting original documents in an electrophotographic printing machine so as to be selectively opaque or to be capable of forming indicia thereon.